With a background of the recent increase in communication traffic, the demand for optical communication transmission equipment is increasing. Not only for optical repeating nodes that are being introduced in backbone networks, but also recently introduction of optical communication transmission equipment is being actively performed for local networks. Furthermore optical networks are formed for subscriber loops. Such optical communication systems play an important role with respect to world information networks.
As a typical optical communication system, the optical amplifier repeating transmission system in which for example optical repeating nodes provided with a WDM optical amplifier such as an erbium-doped fiber amplifier (EDFA) are arranged on a transmission line, to thereby give low cost and high reliability, and realize transmission over long distances with large capacity, is mainstream.
In such an optical amplifier repeating transmission system, if the repeating length between nodes becomes long, losses on the transmission line increase. Furthermore, in the case where optical components with various functions are arranged on the light transmission line, losses in the functional optical components are added so that the repeating losses become even greater. Therefore, the input level of the signal light to the WDM optical amplifier of each optical repeating node becomes less, and the optical signal to noise ratio (OSNR) that expresses the intensity ratio of the signal light to the noise light drops, so that there is a possibility of a deterioration in transmission characteristics.
As one means for avoiding this deterioration in transmission characteristics due to the increase in repeating losses, there is known for example a distributed Raman amplifier (DRA) that supplies pump light to a transmission line arranged on an input side of a WDM optical amplifier, and Raman amplifies by using amplification due to a stimulated Raman scattering affect on the signal light propagated on the transmission line. In a system that uses this distributed Raman amplifier, by increasing the input level to the WDM optical amplifier, the OSNR is increased and the transmission characteristics improved, so that the number of spans that can transmit the WDM signal light is increased. Consequently, the distributed Raman amplifier is effective as a means for avoiding the deterioration in transmission characteristics, and has reached a stage of practical use.
FIG. 17 is an example of a common WDM optical communication system that uses a distributed Raman amplifier. Here a transmitter section (Tx) 501 and a receiver section (Rx) 502 are connected by a transmission line 503, and optical amplifiers 504 such as EDFAs are arranged at a required spacing on the transmission line 503. Furthermore, pump light sources 505 for injecting pump light for Raman amplification are provided on the transmission line 503 of each repeating section. Accordingly to such a system configuration, the WDM signal light on the transmission line 503 of the respective repeating sections is distributed Raman amplified by the pump light from the respective pump light sources 505, so that the losses are compensated. As a result the transmission characteristics of the WDM signal light that reaches the receiver section 502 are improved.
Incidentally, for the distributed Raman amplification, it is known that due to wavelength dependency of the Raman gain, a wavelength deviation occurs in the output light power after amplification. This wavelength deviation of the output light power in the distributed Raman amplification, in a WDM optical communication system as illustrated in FIG. 17, accumulates while being increased by the optical amplifiers 504 (in line amplifiers) of the respective repeating sections. Therefore, as illustrated schematically at the top of FIG. 17, a large wavelength deviation occurs in the power of the WDM signal light that reaches the receiver section 502. This accumulation of the wavelength deviation of the signal light power may deteriorate the transmission characteristics of the WDM signal light, due to factors such as nonlinear phenomena of the transmission line, OSNR deterioration, or exceeding the input range of the receiver.
This wavelength deviation of the output light power of the Raman amplifier may be reduced by having a plurality of pumping wavelengths for the Raman amplifier, and appropriately adjusting the pump light power at each pumping wavelength. However an increase in the pumping wavelengths makes the optical circuit configuration and the control mechanism for the Raman amplifier complicated, and hence there is a problem of an increase in cost of the Raman amplifier. To address this problem, for example as illustrated in FIG. 18, there is known a method where a gain equalizer (GEQ) 506 having a fixed loss wavelength characteristic such as to negate the gain wavelength characteristic of the Raman amplifier, is inserted in each of the repeating sections (for example refer to Japanese Laid-open Patent Publication No. 2002-76482).
However, for the conventional method that uses the above gain equalizer, there is a demerit in that for example due to system requirements such as the kinds of transmission lines, the number of wavelengths of the WDM signal light, and the setting of the Raman gain, the gain wavelength characteristics of the Raman amplifier change significantly, and hence it is difficult to reliably compensate the wavelength deviation of the output light power of the Raman amplifier by the gain equalizer.
More specifically, in the case where the types of transmission lines are different, the effective core cross-section areas differ due to the transmission line type, and the situations in which the non linear phenomenon occurs corresponding to the effective core cross-section areas differ. Therefore the efficiency of the Raman effect differs significantly. For example, if a typical single mode fiber (SMF) and a non-zero dispersion shifted fiber (NZ-DSF) are compared, then as illustrated at the top of FIG. 19 the NZ-DSF with the small effective core cross-section area obtains a higher Raman gain efficiency than the SMF with the large effective core cross-section area. Gain efficiency is defined as “gain/pump light total power”. Therefore, as illustrated at the center of FIG. 19, the Raman gain obtained when pump light of a required power is supplied is greater in the NZ-DSF than in the SMF. If a difference occurs in the Raman gain with respect to the required pump light power, then as illustrated at the bottom of FIG. 19, the wavelength difference of the Raman gain (output light power) increases in proportion to the Raman gain.
FIG. 20 schematically illustrates the Raman gain for the case where three pump lights Lp1, Lp2 and Lp3 with different wavelengths are supplied to the transmission line. In this case, the wavelength characteristics of the Raman gain become a characteristic where the gains corresponding to the respective pump lights are overlapped, and the wavelength difference of the output light power expands as the mean gain set in the Raman amplifier becomes larger. Such a wavelength difference in the output light power of the Raman amplifier changes with the system requirements such as the types of transmission lines that are connected. Therefore it is not easy to compensate for this change with a gain equalizer in which the loss wavelength characteristics are fixed.
In order to address the defects in the above conventional method, it is effective to accurately execute control that makes the gain of the Raman amplifier constant, and suppress changes in the gain wavelength characteristics to a minimum. In order to realize gain constant control of a Raman amplifier corresponding to various types of system requirements, the Raman amplifier must have an optical circuit structure and control mechanism that can supply pump light across a wide range from minute power to excessive power, to the transmission line.
That is to say, the necessary pump light power to obtain the required Raman gain, as illustrated at the middle of FIG. 19, changes significantly with the type of transmission line. Furthermore, this also changes corresponding to the number of wavelengths of the WDM signal light. Moreover, there is the possibility that this also changes due to a generation status of the optical loss in the vicinity of the end of the transmission line on the pump light input side. Examples of this optical loss in the vicinity of the end of the transmission line on the pump light input side include for example optical connector loss, loss due to pressing or twisting of the optical fibers acting from the outside, or loss due to the structure or material of the optical fiber. In order to realize gain constant control of the Raman amplifier assuming the above various factors, increasing the power adjustment range of the Raman amplifier pump light is an essential condition.
However, as one factor that prevents an increase in the power range of the Raman amplifier pump light, there is the unstable oscillation operation of the pump laser, which is a problem for the realization of gain constant control of the Raman amplifier. For the pump laser that is normally used as a pump light source for the Raman amplification, it is known that in the vicinity of the oscillation threshold value and in the vicinity of the maximum absolute rating, the output wavelength and the output power both become unstable. That is to say, there is a limit to the lower limit and the upper limit for the range where the pump laser oscillates stably. Raising this upper limit enables better correspondence to high output type pump lasers, but there is a problem as to how far the lower limit can be lowered.
In relation to the unstable oscillation operation on the low output side of the pump laser, it is known that in the light emission region in the vicinity of where the oscillation threshold value is exceeded, a mismatch occurs between the gain peak wavelength and the oscillation wavelength of the pump laser, so that multi-mode oscillation and single mode oscillation occur irregularly, and the oscillation operation becomes unstable. In order to avoid this unstable oscillation operation, the lower limit of the pump light power must be set to a level where it is never low, corresponding to the characteristics of the individual pump lasers. In a case where this lower limit level is lowered, there is a high possibility that the pump laser becomes unstable oscillation operation, so that it is likely that a desired spectrum or power cannot be obtained for the pumping light for Raman amplification.
When the pumping wavelength fluctuates due to unstable oscillation characteristics of the pump laser, the spectrum of the Raman gain also becomes unstable, and a wavelength deviation of the output light power of the Raman amplifier occurs. Furthermore, when the pump light power fluctuates due to unstable oscillation characteristics of the pump laser, then there is a time where the desired Raman gain cannot be obtained, and the improvement effect of the OSNR due to Raman amplification that is assumed at the time of the system design cannot be obtained. Consequently, even considering the unstable oscillation characteristics of the pump laser, there is a demand to increase the adjustment range of the pump light power (in particular to lower the lower limit), and to realize gain constant control of the Raman amplifier.
In a general pumping configuration for the distributed Raman amplifier, for example as illustrated in FIG. 21, the respective pump lights output from the plurality of pump light sources 511-1 to 511-4 with different oscillation wavelengths, is combined and supplied to the transmission line 503, and Raman amplification is performed with respect to all of the wavelength regions of the WDM signal light Ls. Furthermore, in order to obtain a large pump light power, the configuration is such that pump lasers of a high output type are employed as the respective pump light sources 511-1 to 511-4, and pump light pairs with adjacent wavelengths are polarized and combined by polarization beam combiners 512-1 and 512-2. The output power of the respective pump light sources 511-1 to 511-4 is controlled by a control circuit 515, so that each of the pump lights that are combined by the polarization beam combiners 512-1 and 512-2 is combined by a multiplexer 513 and then supplied to the transmission line 503 via a multiplexer 514.
In the pumping configuration such as in FIG. 21, if the output power from each of the pump light sources 511-1 to 511-4 is less than the lower limit, the before-mentioned unstable oscillation operation occurs. That is to say, in a general pumping configuration for a distributed Raman amplifier, the configuration is such that consideration is given to raising the upper limit of the aforementioned pumping light power, so that lowering the lower limit becomes a problem. In order to avoid the occurrence of the aforementioned unstable oscillation operation, it is effective for example, to not drive a part of the plurality of pump light sources 511-1 to 511-4, and to drive the remaining pump light sources at an output power not less than the lower limit, and thus lower the total power of the pump light Lp for supply to the transmission line 503.
However, in the case where, as above, a part of a plurality of pump light sources is made non-driven, the wavelength deviation of the WDM signal light power after Raman amplification increases. Furthermore, a problem where the polarization dependency of the Raman gain increases occurs. This problem will be explained specifically while referring to FIG. 22.
As illustrated at the top of FIG. 22, in the case where all of the four pump light sources 511-1 to 511-4 are driven at a predetermined output power, a Raman gain is obtained having; a peak corresponding to the pump light on the short wavelength side that has been output from the pump light sources 511-1 and 511-2 and polarization combined by the polarization combiner 512-1, and a peak corresponding to the pump light on the long wavelength side that has been output from the pump light sources 511-3 and 511-4 and polarization combined by the polarization combiner 512-2. By amplifying the WDM signal light in accordance with the wavelength characteristics of this Raman gain, a wavelength deviation occurs in the signal light power of the respective wavelengths. However, this wavelength deviation in the signal light power is normal, and is compensated for by application of a gain equalizer 506 in which the loss wavelength characteristics are designed so as to cancel the wavelength characteristics of the Raman gain. Here, in order to simplify the explanation, the output powers of the respective pump light sources are made the same. However actually, considering the wavelength arrangement of the respective pump lights and the inter-signal Raman effect and the like, the powers of the respective pump lights are often different.
On the other hand, in the case where for example, of the four pump light sources 511-1 to 511-4, the pump light sources 511-1 and 511-2 on the short wavelength side are not driven and the pump light sources 511-3 and 511-4 on the long wavelength side are driven at a required output power, so that the total power of the pump light Lp supplied to the transmission line 503 is lowered, then as illustrated at the center of FIG. 22, in the wavelength characteristics of the Raman gain, the peak on the short wavelength side disappears, and there is only the peak on the long wavelength side. In this state, the wavelength deviation of the signal light power after Raman amplification increases and the loss wavelength characteristics of the gain equalizer 506 become incompatible. Therefore a transmission error occurs in the signal light on the short wavelength side.
Furthermore, in the case where for example, one of the two pump light sources where the pumping wavelengths are adjacent (here, the pump light source 511-4 of the pump light sources 511-3, 511-4 on the long wavelength side) is not driven, then as illustrated at the bottom of FIG. 22, in the wavelength characteristics of the Raman gain, the peak on the long wavelength side is lower than the peak on the short wavelength side. Also in this state, the loss wavelength characteristics of the gain equalizer 506 become incompatible. Therefore a wavelength deviation occurs in the signal light power after passing through the gain equalizer 506, and together with this, a bias occurs in the polarization state of the pump light Lp supplied to the transmission line 503. Consequently the polarization dependency of the Raman gain is also increased.
Considering this problem, in the general pumping configuration as illustrated in FIG. 21, conventionally it has been practical to drive all of the pump light sources 511-1 to 511-4, and to increase and decrease the output powers of each uniformly at the same time. For this it is convenient to raise the upper limit of the output power of the pump light sources 511-1 to 511-4, and make the pump light power supplied to the transmission line 503 a maximum. However, even if the individual pump light sources 511-1 to 511-4 are driven at the lower limit of the output power, a large number of pump lights are combined and supplied to the transmission line 503. Therefore the total power of the pump light Lp becomes great, so that the effect of lowering the lower limit is not obtained. That is to say, lowering of the lower limit of the pump light total power supplied to the transmission line 503 becomes a problem.
In a state where all of the pump light sources 511-1 to 511-4 are driven, as one technique for lowering the total power of the pump light Lp supplied to the transmission line 503, a configuration is considered where, for example as illustrated in FIG. 23, a variable optical attenuator (VOA) 516 is inserted on the pump light optical path positioned between the two multiplexers 513 and 514. In this configuration, each of the pump light sources 511-1 to 511-4 is oscillation operated stably by driving at an output power not less than the lower limit, and the power of the pump light Lp for which the respective pump lights have been combined into one by the polarization beam combiners 512-1 to 512-2 and the combiner 513, is attenuated by the variable optical attenuator 516. As a result, pump light Lp with a small total power can be stably supplied to the transmission line 503.
Moreover, as another technique to that described above, for example there is proposed a technique to provide an erbium-doped fiber (EDF) on the output side of the Raman amplification pump light source, and use the absorption property of the EDF to avoid the unstable oscillation characteristics of the pump laser (refer for example to Japanese Laid-open Patent Publication No. 2008-164836). In this conventional technique, by using the physical phenomena where when the pump light power input to the EDF is large, the absorption of the EDF becomes small, while when the pump light power is small, the absorption of the EDF becomes large, the pump light source is operated stably at the time of minimum gain, and the variable range for the Raman gain is extended.
However, in the configuration illustrated in FIG. 23 that employs the variable optical attenuator, in the case where it is necessary to supply pump light Lp of a large total power to the transmission line 503, the output power of the respective pump light sources is wastefully consumed due to the insertion loss of the variable optical attenuator 516, and hence this becomes a factor for limiting the upper limit of the supply power of the pump light Lp to the transmission line 503. Therefore, there is a problem in that the adjustment range that includes both the upper limit and the lower limit of the pump light power cannot be extended.
Furthermore, in the conventional technique where the above EDF absorption property is used, there is a problem in that losses of pumping system increase when the pump light power is large. That is to say, compared to when the pump light power is small, when the pump light power is large, the EDF absorption becomes comparatively small, and the very existence of the EDF acts as a loss medium irrespective of the magnitude of the power with respect to the output light from the pump light source. Moreover, the absorption of the pump light in the EDF changes due to slight modifications to the erbium doping density or the EDF length. Therefore it is necessary to also consider the individual variations in the absorption of the EDF that is actually connected to the output side of the pump light source. Furthermore, the mode field diameter of the EDF is different to the mode field diameter of the single mode fiber that is generally associated with the usual optical components. Therefore it is necessary to allow for losses at both ends of the EDF (approximately 0.5 dB×2 places) when optical components other than the EDF are fusion spliced. If the above loss generated by providing the EDF on the output side of the pump light source, and variations in this loss are considered, then compared to the case where the EDF is not provided, it is necessary to prepare a pump light source that obtains a higher output. Therefore this invites a cost increase for the Raman amplifier.
Moreover, due to the pump light receiving the losses due to the absorption property of the EDF, the driving current for the pump light source necessary for realizing the required gain is increased. Therefore there is a problem in that the power consumption of the Raman amplifier is increased. Furthermore, there is also a problem of installation space for the EDF inside the Raman amplifier. More specifically, the EDF with a length of several meters and the two splice securing tubes that are applied to both ends of the EDF are added to the configuration of the existing Raman amplifier. Normally, since a Raman amplifier uses a pump light source of a high output, it is necessary to ensure a wide radiation space compared to for the other components. Taking this point into consideration, the abovementioned addition of optical components is unsuitable in packaging design, and in order to enable packaging, enlargement of the Raman amplifier cannot be avoided.
In order to realize an extension of the adjustment range for the pump light power, it has also been considered to respectively develop; an pump laser that has been made (designed) adjusted for a large pump light power region, and an pump laser that has been made (designed) adjusted to a small pump light power region, and to also prepare gain equalizers corresponding to each of these, and to suitably select and install the pump laser and the gain equalizer in accordance with the type or environmental conditions of the transmission line. However, in this technique, the variety of pump lasers and gain equalizers corresponding to the types of transmission lines, becomes extremely large. Therefore, their management becomes extremely complex so that this is not practical.
Furthermore, it has also been considered to apply a tilt monitor for detecting the wavelength characteristics of the Raman gain, and a gain equalizer in which the loss wavelength characteristics are changeable, and to suppress the wavelength deviation of the signal light power after Raman amplification, to thereby lower the lower limit of the pump light power. However, in this technique, due to the insertion loss of the tilt monitor and the variable gain equalizer, the transmission characteristics such as OSNR of the WDM signal light deteriorate significantly, so that it becomes necessary to increase the Raman pump light power in order to compensate for the insertion loss. Therefore, the above technique is also impractical from the view point of; use of a higher output pump light source, an increase in power consumption, and the complexity of the optical circuit configuration and the control mechanism.